Light-sensitive silver halide print-out emulsions



United States Patent I O 3,409,436 LIGHT-SENSITIVE SILVER HALIDE PRINT-OUT EMULSIONS Francis J. Farren, Vestal, and Jerome Sklute, Kirkwood, N.Y., assignors to GAF Corporation, a corporation of Delaware No Drawing. Filed Apr. 1, 1965, Ser. No. 444,817 14 Claims. (Cl. 96-94) ABSTRACT OF THE DISCLOSURE Process for preparing print-out emulsions by preparing a gelatin emulsion of the silver salt of an aliphatic hydroxy-polycarboxy acid, converting the silver salt to a silver halide, and sequentially adding a water soluble iodide, a water soluble lead salt and a water soluble bromide.

This invention relates to light-sensitive silver halide print-out emulsionsv suitable for use in oscillograph recording instruments wherein said light-sensitive emulsion is prepared by the process comprising treating a composition comprising gelatin and a silver salt of an aliphatic hydroxypolycarboxylic acid in sequence with a watersoluble halide salt other than the iodide, followed by treatment with a water-soluble iodide salt, a water-soluble lead salt, and a water-soluble bromide salt.

As pointed out in commonly assigned copending application Ser. No. 371,814, filed June 1, 1964, oscillograph recording devices are instruments wherein signals from an external source are transformed as by mirror galvanometers or equivalents into oscillating beams of light. These beams of oscillating light are focused through an optical system onto the recording paper. The light source commonly used in oscillographic recording instruments is a high pressure mercury vapor lamp and the light impinging on the sensitive paper at the point of recording is normally quite intense. Hence, the emulsion must be sensitive to this type of illumination. The barely visible latent image of the recording papers is usually latensified or increased in density by overall exposure to ambient light of low intensity, preferably fluorescent or ultraviolet light, to yield a strong and fully visible print-out image. This latensified image is usually and preferably in the 0.02 to 0.6 reflectance density range.

In view of the double exposure, print-out emulsions of necessity must exhibit a pronounced sensitivity to exposures of high intensity and short duration, and a very low sensitivity to exposure of low intensity and long duration. Preferably these low intensity exposures of long duration should have a desensitizing effect on the unexposed silver halide in background areas. In this way the background fog level remains relatively low during the latensification procedure and also during subsequent examination of the latensified material under ambient light.

Print-out emulsions for recording oscillographs are well known in the art and have been described, for instance, in US. Patent 3,039,871. The silver halides employed in the above-described print-out emulsions normally have been combined with a mixture of lead iodide and lead bromide in a comparatively slow and time consuming colloid milling operation.

In commonly assigned application Ser. No. 371,814, filed June 1, 1964, there is provided a simpler method of preparing print-out emulsions which does not necessitate the utilization of special equipment. While these print-out emulsions are highly sensitive to exposures of high intensity and short duration and exhibit relatively little fog during latensification, it is desirable to provide print-out emulsions of even greater sensitivity to exposures of high intensity and short duration. The object of this invention is to provide print-out emulsions having all the desirable attributes of application Ser. No. 371,814 and markedly greater sensitivity than the print-out emulsions of application Ser. No. 371,814.

We have now found that the objects of our invention can be attained by forming a mixture of gelatin and silver salt of an aliphatic hydroxypolycarboxylic acid and adding thereto in sequence a water-soluble halide salt other than the iodide, followed by treatment with a watersoluble iodide salt, a water-soluble lead salt, and a watersoluble bromide. Print-out emulsions prepared in this manner have all the desirable attributes of the Ser. No. 371,814 application and in addition are markedly more sensitive, yield denser images, and are more stable to ambient light after exposure and latensification. The increased sensitivity and net density of the print-out images of this invention are believed to be due to the fact that print-out images are formed in the interior of the silver halide crystals and that the preparation of the silver halide crystals in the described manner results in a high concentration of internal imperfections within the silver halide crystals. It is believed that the more imperfections in the silver halide crystals, the greater the sensitivity of the crystal.

In the process of our invention there is first formed a silver salt of a hydroxycarboxylic acid. Part or all of the hydroxypolycarboxylic acid moiety of the salt is replaced by adding halide salts thereby forming silver hydroxypolycarboxylic acid bromo-iodide crystals or silver hydroxypolycarboxylic acid chloro-bromo-iodide crystals in two or more steps. In this way silver halide crystals containing a large number of internal imperfections are formed which exhibit high photolytic sensitivity.

Suitable aliphatic hydroxypolycarboxylic acids which can be utilized in this invention include citric acid, oxalic acid, tartaric acid, etc. The various water-soluble salts of these acids are also deemed to be included in the term aliphatic hydroxypolycarboxylic acids and/or in the individualterms, citric acid, oxalic acid, etc. Suitable salts include the alkali metal salts such as sodium, potassium and lithium, ammoniumsalts, etc. Best results have been obtained using salts of citric acid.

In general, the emulsions of our invention are prepared by providing an aqueous gelatin-aliphatic hydroxypolycarboxylic acid mixture and adding thereto a suitable water-soluble silver salt, preferably silver nitrate. If desired, the aqueous gelatin hydroxypolycarboxylic acid composition may contain various water-soluble halide salts such as sodium bromide, potassium bromide, sodium iodide, potassium iodide, sodium chloride, potassium chloride, etc. However the hydroxypolycarboxylic acid must be present in a concentration sufiicient to provide at least ten percent of the total anionic equivalents provided by the hydroxypolycarboxylic acid and halide ions. In the event that the hydroxypolycarboxylic acid provides less than ten percent of the anionic equivalents provided by the polyhydroxycarboxylic acids and halide salts an in suificient concentration of silver citrate is formed in the subsequent addition of the silver salt. This is due to the fact that halide salts react preferably with silver salt prior to the reaction of the hydroxypolycarboxylic acids. As pointed out above, it is essential that some silver citrate be formed in addition to the silver halide, and on subsequent addition of halide salts, the silver salt of the hydroxypolycarboxylic acid be replaced in the silver salt crystals resulting in internal imperfections.

Best results have been obtained with from 15 to 40 equivalent mole percent of the hydroxypolycarboxylic acid.

After the formation of the silver hydroxypolycarboxylic acid salt, the hydroxypolycarboxylic acid salt is removed from the crystal and replaced by the addition of halide salts. After the formation of silver halide crystals having a high degree of internal imperfections, there is added thereto in sequence a water-soluble iodide salt, such as an alkali metal iodide (sodium iodide, potassium iodide), a water-soluble leadsalt, such as lead nitrate or lead acetate, followed by the addition of a water-soluble bromide salt, such as alkali metal bromide (sodium bromide, potassium bromide).

As pointed out in the aforesaid copending application, the addition of the iodide ion is essential for the proper photolytic print-out reaction. The residual or background sensitivity of these silver halide crystals to illumination of low intensity, such as ambient light, is reduced by treating the emulsions with the water-soluble salt of a divalent lead ion, Pb++ followed by a treatment with a water-soluble alkali metal bromide as described above. The overall results, then, is the formation of a print-out emulsion system which is eminently suitable for the recording of oscillograph traces or high intensity beams of light from the above-mentioned sources, including high intensity incandescent light sources which provide electromagnetic'energy in the visible and ultraviolet range.

The improved oscillograph recording print-out emulsions when coated on a suitable support, such as paper, have a higher fresh net density, a faster latensification rate, a greater stability and a better sharpness than the emulsions described by the prior art. Further, as indicated above, the time consuming colloid milling operation is not needed for the blending of the basic ingredients.

This product also has the advantage that the amounts of silver salts used is rather moderate. A considerable saving in silver can be achieved because the emulsion can be coated on record paper to a (metallic) silver weight of about 2.0 to 4.0 grams per square meter to produce an entirely satisfactory record. The record paper prepared with the emulsion of the present invention also can readily be permanized by processing in special developers containing a developing agent and a silver halide solvent to render the image and the background completely insensitive to further illumination.

The type of gelatin, the silver to gel ratio, the hydroxy polycarboxylic acid used, the silver chloro-bromide-iodide ratio, the concentration of reagents, the pH and the bromide content of the emulsion, the addition times and the temperatures all may be optimized to yield the highest trace print-out density, the fastest latensification rate and the lowest background fog level consistently obtainable. It is possible, however, to deviate considerably from the detailed specifications given below without causing serious variance in the qualities and characteristics of the emulsion.

The preferred and limiting values of the various factors, materials and operating conditions are listed below as follows:

(1) Inert gelatin is preferred to active or sensitizing gelatin in order to reduce the surface sensitivity during the ripening.

(2) An ammonia type emulsion is preferred to the boiled type because this permits the attainment of an average silver halide crystal size of approximately 0.5 to 5.0 microns. This is considered near optimum for this type of record emulsion.

(3) The making temperature can be varied considerably, preferably between about 40 C. and 60 C.

(4) The silver halide-hydroxypolycarboxylic equivalents ratio of the starting emulsion can be varied all the way from 100 equivalent percent silver salt of hydroxycarboxylic acid to 90 percent silver halide.

(5) The pH of the ripened emulsion is lowered and adjusted to a value below 7.0 and preferably to a value between 1.8 and 5.0 to reduce background fog level. Any suitable acid, such as sulfonic acid, citric acid or tartaric acid may be used for this purpose. pH values above 7.0 are undesirable since they tend to increase fog density.

(6) The ripening temperature can be varied from about 40 C. to 70 C. and the total ripening time may be varied between about minutes and 2 hours. As previously indicated, the emulsion should be ripened with the addition of alkali metal iodide, soluble lead salt and alkali metal bromide in the sequence just stated. 7

(7) The quantity of alkali metal iodide which is added as a ripening final can be varied all-the way from 0.5 to mole percent based on the silver content. An optimum is considered about 2.5 to 7.5 mole percent.

(8) The quantityof lead bromide which is incorporated in the emulsion by addition of solublelead salt and soluble bromide salt can also be varied-from.0.5 to.

20 mole. percent. As its lower value, the lead bromide gives the best trace density, but also yields the highest background fog density. Higher values give lower trace densities, but also lower background fog level. About 5. to 12.0 mole percent of lead bromide based on the total silver content of the emulsion appears to be optimum.

(9) The finished emulsion is coated on paper to a coating weight of about 2.0 to 4.0 grams of silver per square meter. When exposed in the oscillograph' or to other high intensity light sources and latensified by exposure with either fluorescent or ultraviolet light of low intensity, this coated paper is capable of yielding a net trace reflectance density of approximately 0.02 to 0.60 unit.

(10) The stability of the coated emulsion can be improved and the fresh net trace density can be increased by an additional 0.1 to 0.2 unit by applying an additional surface coating. The emulsion layer is preferably 'overcoated with a gelatin solution which contains the alkali metal salt of a weak acid (20.0 grams to 40.0 grams of sodium formate, sodium acetate or borax per liter of 2.0 percent surface gel). These salts, in combination with the previously described acids, form a buffering system. The preferred alkaline ingredient is sodium formate.

The above print-out emulsions can be optically sensihydroxyethyl cellulose or polyvinylpyrrolidone can be used.

The following examples are illustrative and should not be construed as limiting the scope of our invention.

Example I This example illustrates the preparation of a preferred print-out emulsion of the type described in copending application, Ser. No. 371,814. An aqueous'silver nitrate composition comprising 98.4 grams silver nitrate, 17.0 ml. nitric acid (20% by weight nitric acid), 90.4 ml. ammonium hydroxide (28%) and 160 ml. of water was added to 20.3 grams sodium chloride, 33.9 grams potassium bromide, 6.1 grams gelatin in 278.4 ml. water, which had been adjusted to C. After the addition of the silver nitrate composition was completed, the mixture was digested at 48 C. for 10 minutes. Sixty-seven grams of potassium bromide was then added to the emulsion to convert the silver-chloro-bromide into silver bromide. A 50% byweight ammonium sulfate solution was added to the aqueous composition in order to precipitate out the silver halide. The precipitate was washed 4 times with water and reconstituted with 55.4 grams gelatin and sufficient water to bring the weight of the composition to 492 grams. The gelatin was permitted to swell for 30 minutes, heated to C., cooled to 40 C. and digested at this temperature for 40 minutes. At this point the pH of the emulsion was adjusted by adding 49.2 ml. of sulfamic acid and the silver bromide was converted to silver-bromoiodide by the addition of 49.2 ml. of 10% by weight potassiurn iodide solution. After this composition was adjusted t o 5.1:.fC. anddi'gested for 30 minutes, 98.6 ml. of

.by weight lead nitrate was added. After a short diges- Example II This example illustrates the preparation of one of the preferred emulsions of this invention wherein essentially pure silver citrate is formed prior to the addition of hal de salts. An aqueous silver nitrate composition comprismg 160 ml. water, 98.4 grams silver nitrate, 17.0 ml. nitric acid (20% by weight) and 98.4 ml. of ammonium hydroxide (28%) was added to a composition comprising 58.4 grams sodium citrate, 6.1 grams gelatin and 278.4 ml. of water, which was maintained at 48 C. The resulting silver citrate was converted to silver bromo-chlorocitrate by the addition of 17.2 grams potassium bromlde and 27.0 grams sodium chloride in 128 ml. water. There was then added an additional 55.2 grams potassium bromide in 123.2 ml. water. The resulting composition was digested at 48 C. in the manner described in Example I for 10 minutes, cooled to 42 C., precipitated with ammonium sulfate, washed 4 times with water and added thereto 50.4 grams of gelatin and sufiicient water to bring the weight of the composition to 492.0 grams. The gelatin was permitted to swell for 30 minutes, heated to 55 C., cooled to 40 C., digested at 40 C. for 40 minutes and adjusted with 49.0 ml. by weight of sulfamic acid. The silver halide was then converted to essentially all silveriodo-bromide by the addition of 29.2 ml. of 10% by weight potassium iodide solution and digested at 51 C. for 30 minutes. There was then added 98.6 ml. of 10% by weight lead nitrate solution and after a short time there was added 112 ml. of a 10% by weight potassium bromide solution. The print-out emulsion was applied to a baryta coated paper stock.

Example III This example illustrates the preparation of a print-out emulsion of this invention wherein there is initially formed a silver-iodo-brorno-chloro-citrate having over 50 equivalent percent of the total anions provided by the hydroxypolycarboxylic acid and halide ions furnished by the hydroxypolycarboxylic acid. The emulsion was prepared in essentially the same manner as in Example II utilizing 278.4 ml. of water, 42.0 grams sodium citrate, 4.0 grams potassium bromide, 1.0 gram potassium iodide, 8.0 grams sodium chloride and 6.1 grams of gelatin and the same silver nitrate composition utilized in Example II. The halide concentration of aqueous potassium bromide-sodium chloride composition utilized in Example II was reduced in order to balance the concentration of halide in the silver halide crystals. The composition comprised 128.0 ml. water, 13.2 grams potassium bromide and 19.0 grams sodium chloride. To this composition was again added 55.2 grams of potassium bromide and 123.2 ml. of water. The resultant composition was digested for 10 minutes at 48 C., cooled to 42 C., precipitated with ammonium sulfate, washed 4 times with water and a gelatin emulsion reconstituted by the addition of 55.4 grams gelatin and sufiicient water to bring the weight of the composition to 49.2 grams. The gelatin was permitted to swell for 30 minutes and heated to 55 C. and permitted to cool to 40 C. and digested at this temperature for 40 minutes. The pH of the emulsion was adjusted by the addition of 49.2 ml. sulfamic acid followed by the addition of 49.2 ml. of 10% by weight potassium iodide. After the emulsion temperature was raised to 51 C. and digested for 30 minutes, 98.6 ml. of 10% by weight potassium nitrate was added thereto. After a short period 112 ml. of 10% by weight potassium bromide was added.

The emulsion was then applied to a baryta-coated paper' stock as in the preceding examples.

6 Example IV This example illustrates the high speed of the print-out emulsions of this invention as contrasted to print-out emulsions prepared by starting initially without a silver salt of a hydroxypolycarboxylic acid. The light-sensitive elements prepared in Examples I through III were exposed for 10 second on the EGG Mark VI Sensitometer. The results are set forth below in Table I.

1 Gamma is a measure of gamma from standard H & D curves with a ensity scale expanded five times.

It can be readily seen from the above data that lightsensitive emulsions of this invention have markedly greater sensitivity and give rise to images of higher net density than previously known print-out emulsions.

We claim:

1. The process of preparing light-sensitive print-out emulsion of increased light-sensitivity which comprises the steps (1) providing a composition comprising gelatin and a silver salt of an aliphatic hydroxypolycarboxylic acid, (2) replacing essentially all of the anions in the silver salt provided by the aliphatic hydroxypolycarboxylic acid with halide ions in any order selected from the group consisting of chloride, bromide, iodide and mixtures thereof, (3) adding sequentially to said composition a water-soluble iodide salt, a water-soluble lead salt and a water-soluble bromide salt.

2. The process of claim 1 wherein said silver salt of an aliphatic hydroxypolycarboxylic acid comprises a silver halo salt wherein the aliphatic hydroxypolycarboxylic acid moiety provides at least 10' equivalent percent of the anions provided by halide anions and the aliphatic hydroxypolycarboxylic acid anions.

3. The process of claim 1 wherein said aliphatic hydroxypolycarboxylic acid comprises citric acid.

4. The process of claim 1 wherein said water-soluble lead salt comprises lead nitrate.

5. The process of claim 1 wherein said water-soluble iodide salt and said water-soluble bromide salt are each salts of an alkali metal.

6. The process of preparing a light-sensitive print-out emulsion of increased light-sensitivity which comprises the steps of 1) providing a composition comprising gelatin and a silver salt of an aliphatic hydroxypolycarboxylic acid, (2) replacing essentially all of the anions in the silver salt provided by the aliphatic hydroxypolycarboxylic acid with halide ions selected from the group consisting of chloride. bromide, iodide and mixtures thereof wherein the last halide salt added to said composition comprises a water-soluble iodide salt, (3) adding in sequence to said composition a water-soluble lead salt and a water-soluble bromide salt.

7. The process of claim 6 wherein said silver halide salt of an aliphatic hydroxypolycarboxylic acid comprises a silver halo salt wherein the aliphatic hydroxypolycarboxylic acid moiety provides from 15 to 40 equivalent percent of the anions provided by the halide anions and the aliphatic hydroxypolycarboxylic acid anions.

8. The process of claim 6 wherein said alpihatic hydroxypolycarboxylic acid comprises citric acid.

9. The process of claim 6 wherein said water-soluble lead salt comprises lead nitrate.

10. The process of claim 6 wherein said Water-soluble iodide salt and said water-soluble bromide salt are each salts of an alkali metal.

11. The process of preparing a light-sensitive print-out emulsion of increased light-sensitivity which comprises the steps of 1) providing a composition comprising gelatin and a silver salt of citric acid, (2) replacing essentially all of the anions in the silver salts of citric acid with halide ions'in any order selected from the group consisting of chloride, bromide, iodide and mixtures thereof, (3) adding sequentially to said composition a'water-soluble alkali metal iodide salt, lead nitrate and a water-soluble alkali metal bromide salt.

12. The process of claim 11 wherein said silver salt of citric acid comprises a silver halo citrate wherein the anion portionof the'c'itric'acid' provides'at least 50 eqiiivalent percent of the anions prov'ided'bvhalide anions 'andjc'itric acid anions. r a

13. The process of prpa'ring a light-sensitive print-oiit emulsion of increased"light-sensrtivitywhich comprises-the steps of (1)" providing a composition comprising gelatiri' and a silve'r salt of citric acid; (2)"repla'cirigessentiallv' all of the anions in 'thesilver' salt'providd by the'citric acid with halide ions selected from the"gro1ip' consisting of chloride, bromide, iodide and mixtures'thereofwherei'r'r saidiodide salt is the last halide salt a'dded to said composition, (3) adding sequentially to'said composition 165a- H nitrate and a water-soluble alkali metal i'bromide' saltr 20- 8', 14;The process of claim 13 wherein said silver salt of citric acid"compris'es a silver halo citrate salt wherein the anion portion of the citric acid provides at least 50 equivalent percentof the anions provided by halide anions and citric acid anions.

" References Cited" UNITED STATES PATENTS NORMAN G. TORCHIN, Primary Examiner. J. R. Assistant Examiner. 

